Automatically manage previews
You turn automatic preview maintenance on or off on a project-by-project basis.
Have Aperture automatically update previews for selected projects
In the Library inspector, select the projects whose photos you want to maintain previews for.
Choose Maintain Previews For Project from the Library Action pop-up menu .
A checkmark appears next to the Maintain Previews For Project menu command. Aperture will now automatically update previews when photos are imported into the projects, as well as when versions are created or adjusted.
Note: Turning on automatic preview maintenance does not generate the previews. It only tells Aperture what to do the next time a photo is imported or a version is created or adjusted. To generate previews, you can use the commands described in Manually manage previews.